Method and apparatus for dehalogenating organic halide through electrolysis

ABSTRACT

An object is to provide a method for completely and fully dehalogenating an organic halide through electrolysis, and an electrolysis apparatus used therefor. One embodiment of the invention is a method wherein, when carrying out dehalogenation by electrolyzing an organic halide or a liquid containing the organic halide, a diaphragm electrolytic cell in which an anode and a cathode are separated is used, and electrolysis is carried out while applying ultrasonic waves using ultrasonic wave generating means and stirring using stirring means, and an electrolytic cell used therefor. Another embodiment of the present invention is a method wherein, when carrying out electrolysis, a hydrogen-absorbing metal or alloy is used as a cathode. In this case also, it is preferable to further use a diaphragm electrolytic cell, and carry out electrolysis using ultrasonic wave generating means and stirring means.

TECHNICAL FIELD

The present invention relates to a method for dehalogenating an organic halide through electrolysis so as to render it harmless, and apparatus used therefor.

BACKGROUND ART

Organic halides such as polychlorinated biphenyls (PCBs) or dioxins are generally harmful to the human body, and many thereof pollute the environment. In particular, PCBs cause a serious social problem as an environmental pollutant since they are resistant to degradation, remain for a long period of time without decomposing, and continue to pollute the surroundings. Furthermore, PCBs also have the problem that it is difficult to process them so as to render them harmless when they are disposed of, etc. since they are chemically very stable substances.

With regard to methods that are currently allowed by the Waste Disposal Law regulating the disposal of waste oil, etc. containing PCBs, there are known a high temperature thermal decomposition method, a dechlorinating decomposition method, a hydrothermal oxidative decomposition method, a reductive thermochemical decomposition method, a photodecomposition method, and a plasma decomposition method. Among these, from the viewpoint of especially large-sized and specialist decomposition equipment being unnecessary, no harmful products being formed, the possibility of reuse of by-products, etc. the dechlorinating decomposition method is preferable.

In general, in a dehalogenating decomposition method, by mixing sodium metal, an organic alkali metal, a catalyst, etc. with an organic halide and carrying out a chemical reaction, the halogen in the organic halide is replaced with hydrogen, etc. (ref. e.g. Patent Publications 1 to 3 below). However, there are the defects and problems that the metal and catalyst used in the dehalogenating decomposition are expensive and it is necessary to carry out this chemical reaction in an anhydrous organic solvent.

As a method for making PCBs, etc. harmless by a, relatively simple procedure and means, there has been proposed a method in which reduction/dehalogenation is carried out by adding and mixing calcium metal with an alcoholic solution of a PCB, etc. (ref. Patent Publication 4 below). However, this method also has the problem that it is necessary to use an organic solvent.

The present inventors have also proposed a method in which a PCB and a solvent such as water or acetonitrile are mixed, and the PCB is treated by electrolysis (ref. Patent Publication 5 below). This method is an excellent method since the PCB, etc. can be made harmless relatively simply. However, in order to carry out dehalogenation completely by this method, it is necessary to modify the conditions and equipment for the electrolysis.

-   (Patent Publication 1) JP-A-2002-756 (JP-A denotes a Japanese     unexamined patent application publication) -   (Patent Publication 2) JP-A-2001-269673 -   (Patent Publication 3) JP-A-8-66494 -   (Patent Publication 4) JP-A-2002-265391 -   (Patent Publication 5) JP-A-2002-345991

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

It is an object of the present invention to provide a method for completely and fully dehalogenating an organic halide through electrolysis, and an electrolysis apparatus used therefor.

Means for Solving the Problems

One embodiment of the present invention is a method for dehalogenating an organic halide, wherein when carrying out dehalogenation by electrolyzing the organic halide or a liquid containing the organic halide, a diaphragm electrolytic cell in which an anode and a cathode are separated is used, and electrolysis is carried out while applying ultrasonic waves using ultrasonic wave generating means and stirring using stirring means.

Another embodiment of the present invention is a method for dehalogenating an organic halide, wherein when carrying out dehalogenation by electrolyzing the organic halide or a liquid containing the organic halide, electrolysis is carried out using as a cathode a hydrogen-absorbing metal or alloy.

Yet another embodiment of the present invention is an electrolytic cell for carrying out dehalogenation by electrolyzing an organic halide or a liquid containing the organic halide, wherein the electrolytic cell comprises an anode and a cathode separated by a diaphragm, ultrasonic wave generating means, and stirring means.

BRIEF DESCRIPTION OF DRAWING

(FIG. 1) A schematic diagram of an electrolytic cell of the present invention.

EXPLANATION OF REFERENCE NUMERALS AND SYMBOLS

-   1 Anode -   2 Cathode -   3 Diaphragm -   4 Anode chamber -   5 Cathode chamber -   6 Liquid to be treated -   7 Ultrasonic wave oscillator rod -   8 Stirring means

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is characterized in that, when carrying out dehalogenation by electrolyzing an organic halide or a liquid containing the organic halide, a diaphragm electrolytic cell in which an anode and a cathode are separated is used, and electrolysis is carried out while emulsifying the organic halide-containing liquid using ultrasonic wave generating means, at the same time using the cavitation effect of ultrasonic waves, which forms at normal temperature and normal pressure a vacuum state with a pressure of 1000 atm and a temperature of 5000° C., and stirring the liquid by stirring means. In this case, a method in which the organic halide or the liquid containing the organic halide is put into the cathode side, and electrolysis is carried out while applying ultrasonic waves to the cathode side using ultrasonic wave generating means and stirring on the cathode side by stirring means is more efficient. The application of ultrasonic waves and the stirring are not necessarily carried out all the time, and they may be carried out intermittently according to the situation.

The organic halide that is targeted in the present invention means an aliphatic, alicyclic, aromatic, or polycyclic compound having fluorine, chlorine, bromine, or iodine in the molecule. For example, PCBs and dioxins, which are considered to cause problems that affect the environment, are representative examples.

Electrolysis conditions depend on the state of the target organic halide or a liquid containing the organic halide, but they are usually adjusted to give a voltage range of 5 to 500 V and a current range of 5 to 100 A. However, when the electrolytic cell is large, electrolysis conditions in which the voltage and current are higher are employed accordingly. For electrolysis, use of a direct current power supply is appropriate, but an alternating current power supply of, for example, 50 to 60 Hz, or a high frequency power supply of, for example, 1 KHz may be used. When carrying out electrolysis, in order to increase the electric conductivity, it is preferable to carry out electrolysis by adding to the organic halide, the liquid containing the organic halide, or a liquid to be treated containing the above, a salt such as a hydroxide or a chloride of an alkali metal or an alkaline earth metal. By so doing, a voltage of, for example, 200 V necessary for water is reduced to 1/10, that is, to about 20 V, and the temperature increase of the liquid that is treated can be suppressed, which is desirable. The amount thereof added is 1 to 50 g per L of the liquid to be treated, and preferably 3 to 10 g. It is preferable to carry out electrolysis under conditions in which a hydroxide or chloride salt is added to the anode side and a smaller amount thereof or none is added to the cathode side.

The present invention carries out, as a reduction reaction at the electrolysis cathode, a substitution reaction between a halogen of the organic halide and hydrogen generated by electrolysis of water, etc. However, if the cathode and the anode are not separated by a diaphragm, a reversible reaction proceeds, and there is a possibility that a PCB might be regenerated at the anode as a result of an oxidation reaction. It is therefore desirable to prevent the reversible reaction by preventing the product at the cathode from physically contacting the anode. As a method therefor, a diaphragm electrolytic cell in which an anode chamber and a cathode chamber are separated is used. As the diaphragm, there are an ion-exchange membrane, an organic or inorganic microporous membrane, etc., and an appropriate one may easily be selected therefrom while taking into consideration corrosion resistance, mechanical strength, pore size and distribution, electrical resistance, etc. The shape is not particularly limited and may be any shape as long as it prevents substances that have been generated or are present at the anode and the cathode and have dissolved in an electrolyte from being mixed by diffusion convection. In practice, a diaphragm electrolytic cell having a configuration in which an anode in the electrolytic cell is covered with a cylindrical anion-exchange membrane is convenient.

In the present invention, an organic halide or a liquid containing the organic halide is used in electrolysis as it is or in the form of an organic solvent solution or a mixed liquid (liquid to be treated) with an aqueous emulsion or water. For example, when a PCB is an insulating oil of a capacitor, etc., it may be made into a mixed liquid by adding an appropriate amount of water, made into an emulsion using an appropriate surfactant, or emulsified by, for example, a treatment with ultrasonic waves or microwaves. The same applies in the case of a dioxin, and in the case of, for example, soil contaminated by a dioxin, an appropriate amount of water is added together with a surfactant as necessary, the soil is suspended by mixing while stirring, and it is then subjected to electrolysis as it is or, by removing solids, as a liquid for treatment. When carrying out a reaction, it is of course possible to stir the liquid to be treated by electrical or mechanical stirring means.

In the present invention, when carrying out a dehalogenating reaction through electrolysis, ultrasonic waves are applied to the reaction system using ultrasonic wave generating means such as, for example, an ultrasonic wave generating apparatus. In accordance with the use of such an operation in combination with electrolysis, not only is the cavitation effect due to ultrasonic waves exhibited in the reaction liquid, but also, due to the emulsion effect of ultrasonic waves, micro clusters are formed from the organic halide or from the organic halide and another liquid, thus promoting the reaction more efficiently. As the ultrasonic wave generating means, for example, a known or commercially available ultrasonic wave generation apparatus may be employed. The power of the ultrasonic waves is on the order of 1 to 100 W per L of the liquid to be treated, and preferably 10 to 40 W. In addition to the so-called ultrasonic waves, the ultrasonic wave generating means referred to in the present invention includes means that imparts vibration to an object by frequency such as microwaves.

It is generally convenient to employ room temperature for the reaction temperature, but it may be adjusted in a range from the lowest temperature at which the organic halide or the liquid containing the organic halide does not freeze to the reflux temperature. Furthermore, for example, when electrolysis is carried out at high voltage, the temperature of the liquid to be treated increases, but in such a case the electrolytic cell may be cooled and the liquid to be treated that has been evaporated may be passed through a cooling apparatus and returned to the electrolytic cell. The reaction time depends on the amount to be treated, but it is usually adjusted so as to be in the range of 10 to 100 minutes. It is unnecessary to pay special attention to the reaction atmosphere, and the reaction system may be an open system at normal pressure, but in order to eliminate the influence of air as much as possible, the reaction may be carried out under an inert atmosphere such as nitrogen.

The electrolysis of the present invention is carried out using an electrolytic cell for carrying out dehalogenation by subjecting an organic halide or a liquid containing the organic halide to electrolysis, the electrolytic cell comprising an anode and a cathode separated by the above-mentioned diaphragm, ultrasonic wave generating means, and stirring means. Examples of the anode include platinum, titanium, and carbon. Examples of the cathode include metals such as palladium and titanium, and alloys thereof. The ultrasonic wave generating means may be any type as long as the liquid to be treated is sufficiently emulsified by the emulsion effect and dehalogenation is promoted by the cavitation effect, but it is suitable to use a type in which an ultrasonic wave oscillator rod connected to an ultrasonic wave generator is inserted into the liquid to be treated. The stirring means may employ a known or commercially available stirrer. As a treatment vessel, it is unnecessary to use a special vessel, and in practice electrolysis may be carried out using a general container and apparatus made of, for example, stainless steel, glass, or polyvinyl chloride.

Another embodiment of the present invention is a method in which, when carrying out a dehalogenation reaction by electrolyzing an organic halide or a liquid containing the organic halide, a hydrogen-absorbing metal or alloy is used as a cathode. It is preferable to use a hydrogen-absorbing metal such as palladium or a hydrogen-absorbing alloy such as a titanium-iron alloy as the cathode since hydrogen generated by electrolysis is absorbed by the cathode, subsequently the hydrogen discharged therefrom efficiently replaces a halogen, thus contributing to the dehalogenation reaction, and a reduction reaction is thereby carried out efficiently.

In this case also, it is preferable to employ a method in which a diaphragm electrolytic cell is used for electrolysis, and a more preferred result can be obtained by employing a method in which electrolysis is carried out while applying ultrasonic waves to the liquid to be treated and stirring using ultrasonic wave generating means and stirring means.

Yet another embodiment of the present invention is an electrolytic cell for carrying out dehalogenation by electrolyzing an organic halide or a liquid containing the organic halide, the electrolytic cell comprising an anode and a cathode separated by a diaphragm, ultrasonic wave generating means, and stirring means. The ultrasonic wave generating means and the stirring means may be provided on both the cathode side (cathode chamber) and the anode side (anode chamber), and providing them at least on the cathode side (cathode chamber) is efficient.

An electrolytic cell used for carrying out the electrolysis of the present invention is explained by reference to a drawing. FIG. 1 is a schematic diagram of an electrolytic cell as one example thereof. In FIG. 1, an anode 1 (platinum electrode) and a cathode 2 (palladium electrode) are separated by a diaphragm 3 (anionic membrane). A liquid to be treated 6 is also divided into an anode chamber 4 and a cathode chamber 5 by the diaphragm 3, and from the viewpoint of efficiency a liquid to be treated comprising an organic halide or a liquid containing the organic halide is placed in the cathode chamber 5, and an aqueous solution of a salt such as a hydroxide or a chloride of an alkali metal or an alkaline earth metal is placed in the anode chamber 4. An ultrasonic wave oscillator rod 7 for applying ultrasonic waves to the liquid to be treated 6 is inserted into the cathode chamber 5, and stirring means 8 is placed in the cathode chamber 5. The ultrasonic wave oscillator rod 7 and the stirring means 8 may be inserted into/placed in both chambers. By carrying out electrolysis using the above-mentioned electrolytic cell while applying ultrasonic waves and passing current between the anode and the cathode, a dehalogenation reaction of the organic halide proceeds.

The present invention is explained in detail below by reference to Examples. In the Examples, the concentrations of PCB and dioxin were measured by a GC/MAS method, which is normally employed as an analytical method therefor.

In accordance with the present invention, since a toxic organic halide such as a PCB or a dioxin can be treated at normal temperature and normal pressure, the treatment operation is safe. Furthermore, since these organic halides are decomposed using electricity and water so as to make them harmless, the means and method are relatively simple, inexpensive, and clean in terms of the reaction environment. In the present invention, due to the emulsion effect of ultrasonic waves generated by the ultrasonic wave generating means, micro clusters are formed from the organic halide or from the organic halide and, for example, water. Due to the cavitation effect of ultrasonic waves, an environment with a high pressure of 1000 atm or higher and a high temperature of 5000° C. or higher is formed at the moment a bubble in the cluster bursts. In the present invention, due the emulsion effect and the cavitation effect, dehalogenation through electrolysis is carried out very efficiently.

EXAMPLE Example 1

A liquid to be treated was prepared by mixing 1 L (liter) of a capacitor liquid containing a PCB (PCB concentration, 100 ppm), 10 g of sodium hydroxide, and 20 L of water. The electrolytic cell (treatment vessel being made of stainless steel) of FIG. 1 was used, this liquid to be treated was placed in the cathode chamber, 20 L of an aqueous solution containing 100 g of sodium hydroxide was placed in the anode chamber, a three-phase direct current power source was used as a power source, and electrolysis was carried out at normal temperature and normal pressure for 30 minutes while applying ultrasonic waves. The average voltage of the electrolysis was 20 V and the average current was 50 A. An electric wave rod, which corresponded to the ultrasonic wave generating means, was a metal rod having a diameter of 45 mm and a length of 35 cm, and ultrasonic waves of 20 KHz were applied thereto. The power of the ultrasonic waves was on average 255 W. The applied voltage during electrolysis was substantially constant for 30 minutes, but the current and the power of the ultrasonic waves gradually increased. The liquid to be treated in the cathode chamber was stirred at 1400 revolution/min using a commercial mixer. The PCB concentration in the treated liquid after the electrolysis was 0.5 ppm or less, which was considerably less than the 100 ppm prior to the electrolysis.

Example 2

20 L of an aqueous liquid to be treated containing 500 pg/L of a dioxin was electrolyzed in the same manner and using the same electrolytic cell as in Example 1. The dioxin concentration in the treated liquid after the electrolysis was 0.5 pg or less.

INDUSTRIAL APPLICABILITY

In accordance with the method of the present invention, transformer oil containing PCBs, etc. or another liquid containing PCBs, etc., which have been causing problems in recent years, or incinerated ash or soil containing dioxins can be disposed of by a relatively simple method and apparatus, that is, very economically. Moreover, since the operation is carried out at normal temperature and normal pressure, they can be made harmless by a safe and clean treatment employing water and electricity, and the application value is very high from industrial and environmental viewpoints. 

What is claimed is: 1-10. (canceled)
 11. A method for dehalogenating an organic halide, wherein when carrying out dehalogenation by electrolyzing the organic halide or a liquid containing the organic halide, (1) a diaphragm electrolytic cell in which an anode and a cathode formed from a hydrogen-absorbing metal or alloy, the anode and the cathode being separated by the diaphragm, is used, (2) the organic halide or the liquid containing the organic halide is put into the electrolytic cell on the cathode side, and (3) electrolysis is carried out while applying ultrasonic waves on the cathode side using ultrasonic wave generating means and stirring on the cathode side by stirring means.
 12. The method for dehalogenating an organic halide according to claim 11, wherein the organic halide is a PCB or a dioxin.
 13. The method for dehalogenating an organic halide according to claim 11, wherein an aqueous solution of a salt is put into the electrolytic cell on the anode side, and electrolysis is carried out.
 14. The method for dehalogenating an organic halide according to claim 13, wherein the aqueous solution of a salt is an aqueous solution of a hydroxide or a chloride of an alkali metal or an alkaline earth metal.
 15. A diaphragm electrolytic cell for carrying out dehalogenation by electrolyzing an organic halide or a liquid containing the organic halide, in which an anode and a cathode are separated, wherein the electrolytic cell on the cathode side comprises ultrasonic wave generating means and stirring means, and the cathode is formed from a hydrogen-absorbing metal or alloy.
 16. The diaphragm electrolytic cell according to claim 15, wherein the electrolytic cell comprises a cooling means for the electrolytic cell and a refluxing means for a liquid to be treated that has been evaporated. 